This invention relates, in general, to solid state conductors, and more particularly, to a resonant high conductivity structure made from semiconductor materials.
Superconductivity results in a crystal lattice when electrons which are normally spread throughout energy levels within plus or minus k.sub.B TC of a Fermi energy (E.sub.F) gather in a single energy state which is approximately k.sub.B T.sub.C below E.sub.F, where T.sub.c is the critical temperature at which superconductivity occurs and k.sub.B is the Boltzmann constant. Electrons can only gather in the single energy state by coupling or pairing into what is known as "Cooper pairs". A Cooper pair comprises two electrons with equal and opposite momentum and spin which results in a tighter electron packing density than is possible under normal conditions. Because of the energy gap at the Fermi surface, it takes a finite energy approximately k.sub.B T.sub.C, to break a Cooper pair. Consequently, at temperatures T&lt;T.sub.C. the paired electrons carry charge in their energy level with zero resistance.
According to BCS theory: EQU T.sub.C =1.13.THETA..sub.D e.sup.-1/N(.epsilon..sbsp.F.sup.)V
where .THETA..sub.D is the Debye energy of the material, N(.epsilon..sub.F) is the density of states at the Fermi surface, and V is the interaction strength of the acoustic phonon induced attraction between the two Cooper pairing electrons. Generally speaking, the interactions are weak resulting in low critical temperatures around 1 Kelvin.
The BCS critical temperature formula implies several approaches to improving critical temperature. Increasing frequency of the coupling mode theoretically raises T.sub.c. Besides the acoustic phonons in conventional superconductors, tremendous efforts have been made in searching for new mechanisms with higher energy Bosons, such as optical phonon, plasmon, excition, polaron, magnon, and the like. Another method of raising T.sub.c is by increasing density of states N(.epsilon..sub.F). For bulk materials. however, it is very difficult to increase density of states since N(.epsilon..sub.F) is approximately equal to ##EQU1## where n is the electron concentration. In other words, electrons are used inefficiently in bulk material superconductors because only electrons on the Fermi surface are involved in enhanced conductivity.
Another theoretical method of increasing T.sub.c involves increasing interaction strength via a Boson mode. It should be noted that increasing interaction strength is generally detrimental to increasing frequency of the coupling mode, mentioned above. A high frequency Boson mode results in short retardation time and consequently weaker induced attraction between the two Cooper pairing electrons.
Accordingly, a key feature of superconducting materials is an ability to allow electrons to exist as paired electrons rather than Fermions, which are responsible for normal resistive charge conduction. Much work has been done recently to develop materials in which paired electrons can exist at high temperatures. The present invention deals with a method of promoting the formation of paired electrons at temperatures where such pairing would not naturally occur.
Electrons in a crystal lattice have a characteristic coherence length which is determined by electronic and crystallographic properties of the crystal lattice. External forces such as heat and electromagnetic fields affect this electron coherence length. Electrons in normal conduction states repel each other, and will not come close enough to form pairs. Electrons in superconductors, it is believed, interact with lattice vibrations (phonons) to form pairs. Paired electrons are closer to each other than the electron coherence length. In naturally occurring superconductors electron-phonon interactions result in superconductivity at low temperature, where the electron coherence length is sufficiently long.
The present invention uses semiconductor materials to provide a quantum well structure which is adapted to promote electron-phonon interaction, and in particular, to promote electron-phonon interactions which result in formation of paired electrons in materials and at temperatures where paired electrons do not normally exist.
A similar enhanced conductivity material is disclosed in U.S. Pat. Nos. 5,012,302 and 5,016,970 issued to Herbert Goronkin on Apr. 30, 1991 and Oct. 29, 1991 respectively and assigned to the same assignee as the present invention. U.S. Pat. No. 5,012,302 is incorporated herein by reference.